AJP - Cell Physiology, Vol 253, Issue 1 C137-C146, Copyright © 1987 by American Physiological Society
Altered Ca fluxes and contractile state during pH changes in cultured heart cells
D. Kim and T. W. Smith
We studied mechanisms underlying changes in myocardial contractile state
produced by intracellular (pHi) or extracellular (pHo) changes in pH using
cultured chick embryo ventricular cells. A change in pHo of HEPES-buffered
medium from 7.4 to 6.0 or to 8.8 changed the amplitude of cell motion by
-85 or +60%, and 45Ca uptake at 10 s by -29 or +22%, respectively. The
pHo-induced change in Ca uptake was not sensitive to nifedipine (10
microM), but was Na gradient dependent. Changes in pHi produced by NH4Cl or
preincubation in media at pH values ranging from 6.0 to 8.8 failed to alter
significantly 45Ca uptake or efflux. However, larger changes in pHi were
associated with altered Ca uptake. Changes in pHo from 7.4 to 6.0 or to 8.8
were associated with initial changes in 45Ca efflux by +17 or -18%,
respectively, and these effects were not Na dependent. Exposure of cells to
20 mM NH4Cl produced intracellular alkalinization and a positive inotropic
effect, whereas subsequent removal of NH4Cl caused intracellular
acidification and a negative inotropic effect. There was, however, a lack
of close temporal relationship between pHi and contractile state. These
results indicate that pHo-induced changes in contractile state in cultured
heart cells are closely correlated with altered transsarcolemmal Ca
movements and presumably are due (at least in part) to these Ca flux
changes. In contrast, pHi-induced changes in contractile state appear
principally to involve altered Ca handling within the cell and/or altered
Ca sensitivity of myofibrils.
